Free piston Stirling engines usually drive a mechanical load such as a pump or an electrical alternator. Free piston Stirling coolers are usually driven by an electric or other motor to pump heat from one place to another, for example from the inside to the outside of a freezer cabinet. Due to fluctuations in load power demands for engines and heat transfer demands for coolers, the Stirling machine must have a power control to match the engine's output or the cooler's thermal transport rate to the needs of the system with which the machine is cooperating. For example, a free piston Stirling engine driving a load, such as an electrical alternator, with a varying power demand must increase or decrease engine power output accordingly.
The reason is that, if the load on an engine decreases or cooler thermal transport demand decreases, the amplitude of oscillation of the displacer and piston may increase beyond desirable limits, causing collision of internal engine parts and possible damage. Such overstroke occurs because the energy input to the Stirling machine equals the sum of the energy output and the energy losses. When a load demand decreases, the excess input energy is no longer coupled to that load so it tends to drive the displacer to a higher amplitude. The higher amplitude may be beyond a maximum design amplitude which can result in a runaway condition resulting in a damaging collision. Therefore, it is desirable to limit the amplitude of oscillation of the displacer and piston in the event of a substantial decrease in load demand.
There is, therefore, a need for a means for controlling the amplitude of oscillation of free piston Stirling machines and thereby control the power output of a free piston Stirling engine and the thermal transport rate of a free piston Stirling cooler.